Piping Stress Analysis 12

[ChrisProcess]

Hello, first time poster with a query regarding piping stress analysis.

At the moment I work in plant design Process Engineer. Before issuing drawings we send selected lines out for Stress Analysis (our somewhat simplified criteria for Stress Analysis is D>2", dT>100 Deg C). It is now desired to do this analysis in-house.

I've taken it upon myself to research this.
I've assembled numerous guides (including the CASTI guidebook to ASME B31.3 & Process Piping the Complete Guide by Charles Becht).

I know that Caesar, Autopipe and others are used, but for various reasons they don't want to go this route. Instead they want to establish either a guideline or program that will cover the stress analysis.

We can characterise out piping networks quite easily, in terms of fittings, equipments dimensions, operating conditions etc. via our database system and 3d model.

So what I'm really looking for is somebody that works at this day to day, to give some pointers. I've spoken to my former mechanical engineering lecturer who thinks developing it from the ground up is a bad idea (i.e. go the Caesar route). I would like to get some other opinions on this. If it really is a bad idea, its best to find out at this early stage.

From reading the guides, a lot of it seems pretty vague, or at least up to the designers dicretion.
I would like to know how those working at it proceed and if to develop our own properitary procedures/software is feasible.

I'm assuming for all this that ASME B31.3 is the main guideline to consider regarding Stress Analysis for Process Piping.

Thanks for getting this far!

 

[Sid7]

I am no expert to comment on your work, but this sounds like reinventing the wheel.

I wonder if the employer is happy to spend on design research from scratch (unless this is a educational project), which is already available in the market at competitive prices!!

Good luck.

Siddharth
These are my personal views/opinions and not of my employer's.

 

[ChrisProcess]

Hi Sid7,

I'm thinking you could be right.

What it really boils down to though is that noone here has dealt with stress analysis before. I asked why not go with Caesar and was told the cost was prohibitive. I don't know how much it is, I'm guessing around the €5000 mark (Please, if anyone knows, correct me).

What I'm looking for is a way for us to do Stress Analysis. The method isn't too important, only the end result.

What's involved in using the software and in Stress Analysis overall.

I don't mind getting stuck into the deep and dark calculations in the guides but if it can be avoided then all the better.

I'll be delighted to hear all suggestions, but please provide pros & cons for each.

Many Thanks.

 

[JohnBreen]

Hello ChrisProcess,

Well, this should be interesting. I think that you will get many replies as this board is frequented by many of us who are primarily pressure technology (including piping) engineers. You will also get some replies from folks who are members of various Piping and Pressure Vessel Code Committees. ALSO, you have apparently done some research via looking up previous applicable threads - THANK YOU!!!

I do not think the Guide Books by Glynn Woods and Chuck Becht are very vague - I think they are really rather explicit. However, the Piping Codes do not give you much in the way of design direction - that is intentional as it is not the job of the Codes to guide design. The B31.3 Code assigns most of the responsibility for design (including analysis) to the owner. I assume that you have looked at the scope paragraphs in the Piping Codes and you have decided that B31.3 is the appropriate Code for your company's systems. There can be some variations on that; some companies specify the B31.1 Code be used for the piping in the steam-water loops of boilerhouses. If you have been charged by your company to develop a sort of corporate piping design (and fabrication, erection, examination, testing and repair (fitness for continued service would be included) standard it would not be unreasonable to ask to be sent to a professional seminar on the B31.3 Code (Mr. Woods and Dr. Becht both teach such seminars). Even if you do not intend to do pipe stress analysis in-house, it would be VERY helpful to go to a Caesar II seminar just so that you would be competent in reviewing the formal analyses that are done by outside consultants.

So, when should you do structural analysis (piping flexibility and stress analysis) of any particular piping system? Are you familiar with the B31.3 concept of various fluid services? The fluid service (see B31.3, Appendix M and paragraph 300.2) of the piping system would be one of the variables (the "except for"s that you would want to address in your general dimension-temperature related guideline. It would also be useful to weigh the implications of the consequence of failure against the likelihood of failure in making these decisions (see B31.3, Appendices F and G). Inservices temperature and pressure cycling would also be a factor.

Most larger CPI and HPI corporations have in-house Standards that guide the design of piping systems and in many cases those standards include some guidelines for when a piping system must have a "formal" analysis. Note that the ASME Piping Codes do not require an analysis of every piping system - so in this regard hey are vague. Your D>2", dT>100 Deg C) criteria is not bad for a start but there should be exceptions "allowed". You will of course have to address those exceptions in "your" standard.

If I did not have much experience, I would not want to develop a set of corporate piping design Standards from scratch. Rather I would want to build on the Standards that were developed by others. I am not saying that I would suggest taking another Company's Standards and adopt them by simply changing the company name on the cover. However it would be wise to read through a few of these standards and at least get some idea of what the structure should be (look at the table of contents). You will get a lot of good ideas that you might want in your standards. Also, pay attention to the application of various piping system materials to the specific pressure-temperature services.

Have a look at this:

http://engstandards.lanl.gov/engrman/6mech/pdfs/D20-AppA-ASME_B31.3-r1a.pdf

So, when should you do structural analysis (piping flexibility and stress analysis) of any particular piping system? As I say, this will be an interesting thread. Thank you for posting.

Regards, John.

 

[JohnBreen]

......and

Look at paragraph 319.4.1 of B31.3 (your company really needs to HAVE a copy in their technical library) "319.4.1 Formal Analysis Not Required"

Well, I am not one for "rules of thumb" but you will find some. The books by Rip Weaver (Process Piping Design, Volume one and Volume two) come to mind.

Regards, John

 

[pennpiper]

John Breen,
Please go to

http://www.coade.com/support_discussion.asp

and check your personal messages

 

[ChrisProcess]

Hi JohnBreen,

Thanks for that great response, plenty of things to think about in that.
My saying I found the guides vague is down more to my inexperience than anything else.

I am not a piping designer, so much of the guides content is new to me. The mechanical calculation work falls back to the process department since the pipe designers aren't engineers (The Process Engineers are the closest thing to Mechanical Engineers in the company). So even though we aren't laying out the design we need to figure out how to stress analyze it.....not an ideal situation, I think.

Regarding the ASME B31.3 seminars, I will keep my eyes open, however I'm based in Ireland where these kind of things don't come around too often. Still where theres a will theres a way.

At present, as long as they see "Approved" by the external contractor then they are happy. I guess I'm really driven by this whole thing because a)I don't know enough about it, and b)think things should be done a bit better. At the moment it seems like some dark mystery to everyone.

The LANL manual was one of the first things I came across, and perhaps I should concentrate more on practical applications like this.

I really hope this will be an interesting thread. I'd like to get a discussion going among those that work with Stress Analysis day to day, since I think these are the people with that can give the best direction.

Please keep the opinions coming, and if anyone can give me a rough price for Caesar also that would be great. Thanks.

 

[doberdorks]

I'll add a few thoughts

1. Being able to run the software and being able to do the analysis are not always the same thing.

2. If you have any equipment (vessels, tanks, pumps) or interesting in anyway pipe, rules of thumb may not work.


SLH
PS, cost for Ceasar II should be pretty easy to find...

http://www.coade.com/dealer.asp

 

[KernOily]

I think you are asking for rules of thumb as to when an analysis is required? The B31 codes require ALL pipelines to undergo a flexibility analysis, whether manual or otherwise. The analysis may only consist of a review and opinion by an experienced person that an analysis is not needed, but every line must be looked at to one degree or another.

Since you are in the UK, are you using one of the BS codes, eg BS5500?

Since you asked for rules of thumb, here are a few. A formal stress analysis is required for:

- Liquid lines above 650 F
- All lines above 750 F
- Lines 16" and larger in diameter (e.g. to check for local loads and stresses on pipe wall at supports)
- Lines having substantial concentrated loads such as heavy valves, fittings, unsupported vertical risers and branches
- Lines having local reduction in strength due to the installation of special fittings
- Lines connected to vessels or tanks having appreciable settlement or where there are long vertical runs greater than 30 feet
- Lines with less than standard weight wall thickness
- Lines using non-standard supports and/or having pipe attachments
- All lines connecting to rotating equipment regardless of size
- All lines attached to API 12B bolted tanks larger than 4" NPS
- Lines where corrosion allowance is greater than 1/16" for lines through 4", or greater than 1/8" for all line sizes
- All process, regenerating, and decoking lines to and from fired heaters and steam generators (vibration should be considered for these as well)
- All air-cooled heat exchanger piping (because fin-fans are structurally flimsy)
- All lines with maximum short-term temperature below minus 50 F
- All lines having very long straight runs either horizontally or vertically (definition of 'very long' is in the eys of the beholder)
- All blowdown and flare header systems (forces due to fluid dynamics)
- All multi-phase flow lines (dynamic loads)
- All lines with relief valve with set pressure above 50 psig (thrust load)
- Others: Cast iron lines, FRP, copper, etc.

Of course there are plenty of exceptions to this.

Does this help?

There are manual stress analysis methods out there. One is listed in the book "Piper's Pocket Handbook" by Rip Weaver, available from Gulf Publishing.

Hope this helps! Pete

 

[ChrisProcess]

Hi KernOily and doberdorks, thanks for the comments.

Even though I'm in ireland we still would use the ASME guidelines primarily.

I guess what I'm really looking for, more so than rules of thumb, is a work flow process. What procedure does one follow to perform stress analysis. (e.g. acquire isometrics, define process conditions then build computer model)

I'd like to hear from people in this field as to how they approach it and the process by which they perform stress analysis.

And also in response to doberdorks comment
"1. Being able to run the software and being able to do the analysis are not always the same thing."

I'd like to know what would allow me to use the software to full and proper effect. I'm assuming the software vendor would offer training in the use of the program, but even still a lot of the parameters etc. would be down to the user. What guides/books/areas of study would most benefit correct use of the program?

Thanks.

 

[LSThill]

See John Breen

Basics of pipe stress analysis
thread378-189629

 

[RossABQ]

When I first came into the profession, analysis was done by hand by legions of specially trained designers and engineers. It is certainly possible for someone with the proper knowledge to develop some analysis spreadsheets that take a very quick and macro-level look at bending stresses and pressure stresses in your piping. If you are transporting hazardous or flammable materials, you don't want to be fooling around like that; you will want a program that is certified.

Your costs for CAESAR are only half the picture. Someone needs to be trained to use the program, maintain the program, and interpret the results. That is really the major cost.

My understanding is that you can get a corporate license for CAESAR relatively cheaply and then buy "packets" of run licenses that are something like $10 per run. Once you have some experience the number of runs per system is pretty low. Compared to the labor costs, it is still not the major expense.

I used AutoPipe for years and compared to the CAESAR of the same era, it was a joy to use. Its graphical interface was especially nice. We had an outright license, no per-run cost, but it cost about $15k/year for 6 concurrent users back in the late '90's, including their tech support which pretty much was useless but included free upgrades. It was easy to play with supports on a system to gauge the effects. It is now integrated with their other products and while I have not used it, it would appear you could bury some of the cost of analysis by using AutoPlant for your system designs.

 

[ChrisProcess]

Hi RossABQ,

We actually use Autoplant 3D for our pipe modelling, however for various reasons, they don't really want to deal with Bentley (primarily the annual licenses issue and the poor support in the past). I'm thinking Caesar II will be the choice if we go that route.

I've taken a look at John Breens previous thread and there is a lot of great info in there which I'll assimilate over the coming days. In the meantime if anyone has any further thoughts/opinions I'd like to hear them.
Thanks.

 

[Sid7]

ChrisProcess

I would suggest you to provide more details on your application. The more specifics you will provide, I am sure you will get more detailed response on your query by the most senior and respected members in the industry.

regards,

Siddharth
These are my personal views/opinions and not of my employer's.

 

[ChrisProcess]

Sid7,

Our application would be pharmaceutical piping. Primarily utilities and process fluids. These would include steam, cooling oils, nitrogen, solvents etc. Line sizes would generally range from 1/2" up to 6" (with occasional larger lines). We would also be responsible for specifying and designing various equipment layouts (reactors, pumps, heat exchangers etc.) and the piping to and from these.

What we would like is to perform out own stress analysis rather than contract it out.

I apologise if my initial query was vague, but this is down to my inexperience and lack of knowledge on the subject. Essentially we are starting from scratch, trying to find the best approach for performing our own stress analysis. What I am looking for here is suggestions and direction. I'm happy to say that I've received some very helpful responses so far.

 

[DSB123]

ChrisProcess,
As you are in Ireland and intend or are using ASME how do you ensure the requirements of the PED are met since ASME does not implicitly cover all requiremnts of the PED?

Also why do you want to use Caesar? I know Caesar is used widely but there is a UK based software (PSA5) which is used also in the UK and has excellent user support. Also it covers most Piping Codes and can handle up to 100 load cases. It also automatically calculates the highest stress levels around a bend whereas Caesar only calculates stresses at particular specified points around a bend.
I am not "hammering" Caesar here but want to point out that it's not the "be all and end all" of pipe stress packages and does have limits.

What you need to remember is the Stress analysis software should be considered as a "clever" calculator and should be used as such and in order to achieve a safe and Code compliant system other aspects come into consideration.

I would like to ask if the Process Engineers also have other "hats" such as Instrument Engineer, Electrical Engineer, Civil Engineer etc or is it they feel that Mechanical Engineering is so close to chemical engineering that they can undertake that function without formal training? Remember "horses for courses". I would doubt if a piping stress engineer would consider performing Process engineering tasks!!

 

[ChrisProcess]

PED and ASME B31.3 have different implications. Those lines to be analysed would be part of the site installation. PED would apply to lines within Vendor packages, it's not applicable to these site installations. Stress Analysis is seen as a requirement and B31.3, at least to me, seems to be the standard for dealing with this. Bearing in mind we are not obliged to follow any guideline.

The process engineers generally wear process engineer hats, with occasional costume changes. They don't masquerade as civil engineers or electrical etc. This is a piping issue and it's the lack of dedicated piping & stress engineers (in-house) that has caused the overlap.

I assume this overlap is due to the similarities between process and mechanical engineering (to an extent). Some of the process department are actualy mechanical engineers.
The piping designers have trained in buidling services so wouldn't have the same kind of mechanical background as the process department.

Nobody is assuming that the process department can take the place of trained stress engineers. I'm researching ways to proceed and so I can hopefully offer an infromed opinion on possible directions for the company to take.

 

[UtilityLouie]

I think that everyone seems to be not getting to the issue you're looking at...

I kind of fell into the same position you are in at your company when I started here except that the decision to perform stress analysis in house was already made. The reason it was made was the cost for the program and training was FAR LESS than having a consultant perform the analysis. In other words the pay back was huge.

Since I started, the original engineer that first did stress here has left and I have taken over the reigns. I started out using DOS Triflex. We then upgraded to AutoPIPE for Windows. We recently built a huge stock washing plant with SS piping all running at 140-160F and did stress analysis on almost every pipe on the job - due to SS expansion. One thing that I learned when we started to bring in contract engineers (hired guns for help) is that every engineer has their own program preference and we ended up with a version of Triflex, Caesar and AutoPIPE. The contractor I most admired and felt was the expert of the group liked AutoPipe as well, so I felt better that we were trying to standardize on AutoPipe.

Starting from scratch sure sounds like nothing but problems because the calculations can be complex.

One of the things that I would recommend is that because your employer is shying away from program cost is to use a consultant for stress design for a job or two and have them track costs for JUST THE ANALYSIS separately. You'll find there is a payback there to doing it yourself. That was the path our company took when we were trying to make that decision.

One of the other experiences I've noticed is that if I don't do an analysis job for a while I have to relearn the tips/tricks/requirements (the right way to model supports, anchors, friction, etc). I've also noticed that if you do end up doing an analysis, it is a full time job. You need to do a lot of coordination with the piping designer and structural engineer and it can be an iterative process. The designer puts support locations on the line you're analyzing, you tell them if they are sufficient, you give the loads to the structural engineer and they need to make sure the structure can take what you're dishing out especially when you restrain the pipe. Designing and ordering spring cans takes some iterations as well.

It all takes time and there is no silver bullet. Managers want a program that reads and sucks in the info and drops out a repeatable, canned solution that's easy to implement with no interaction from an engineer. Each job and situation could require a different approach depending on steel design, available space, piping materials/thickness, criticality of systems, etc. You can develop some standards to deal with things, but you can't cover everything.

THE FINAL HURDLE: Getting construction managers and engineering managers and anyone concerned with project cost not to have a heart attack due to the extra money a properly supported and analyzed line costs AND getting them to install the line correctly.

An example: The big job I referenced earlier... The construction manager purposefully tried to keep engineers off site so that we couldn't tell that they weren't installing the supports as designed. The problem was it was a "unit rate" contract and the contractors units didn't include the increased number of specialty supports we called for. The best part was that when they started the systems up, you could tell the systems that weren't restrained or supported correctly. Pipes were running into pipes, there was so much stress on pump suctions that the pumps came out of alignment, lines that we knew were going to be dynamic were hopping around in the rack. We were then called in to diagnose the problem and found all the issues with missing supports (there was a pile of spring cans and guides sitting out in the job lay down yard). It cost 10 times more to fix the job after the fact.

Another example: I did stress analysis on some large bore piping that came down into some large fan pumps (1000 hp). These were important process pumps, alignment was critical and the piping into them could not be designed flexible enough. I ran the analysis made my supporting recommendations and the project manager flipped out due to cost. He then took the piping to a consultant, paid the consultant $75K to reanalyze the lines and the consultant recommended bigger, beefier supports than I did.

Bottom line: Doing the analysis in house can eliminate safety factors that a consultant may use. Maybe a better way to say it is: YOU HAVE CONTROL OVER THE ANALYSIS. If you have enough time to learn and do the analysis, it's cheaper than a consultant. Buy a program. Don't ask which one because 20 different engineers will give you 20 different answers. All of the companies will give you trial versions to try (Caesar, Triflex and AutoPIPE). Make the decision you feel is best and then go to a training class by that software company. Also try to hit a B31.1 or B31.3 class.

Sorry the post was so long, hope it was worth reading...

UtilityLouie

 

[KernOily]

UtilityLouie has some terrific points. But I beg to differ on one and that is that his assertion seems to be one can buy the software, run it, and you're good to go. This is a very dangerous assumption. Do NOT make the mistake that running one's computer stress program makes one a qualified stress engineer. Much time and hands-on experience is required to do this right. A lot of judgment is involved.

One must know the Code being employed, one must understand the generalized 3D state of stress that exists in pipe under various loadings, one must understand the fundamentals behind combined loadings on a beam (bending + torsion + axial), one must understand the fundamentals behind Mohr's Circle and/or the stress transformation equations, one must understand something about failure criteria, e.g. Tresca or von Mises. These are the basics of pipe stress and are crucial whether the stress work is done by computer or by manual methods, and are the skills that set apart the stress engineer from the 'casual user' of TriFlex. This can't be overemphasized. The person that does not know these things is not able to accurately interpret the results from TriFlex/C2/AutoPipe/Whatever.

Maybe there's a reason why the engineering firms have guys who do this full-time?!?!?

We have heavy designers who run stress. They are not degreed engineers. They are -mostly- capable. I am the guy does QA/QC on their work. They get it right about 65% of the time. The rest of the time? Errors caused by misunderstandings of how loads are combined, about how forces and moments can combine to offset each other, how the reactions manifest themselves against restraints or vessel/tank/rotating equipment nozzles, misuse of friction, incorrect modeling of branch connections, misunderstanding of how the Code calculates stresses and stress ranges, etc.

You probably wouldn't want me designing a reactor. Sure, I can read about Kij's, read the Campbell texts, and use software or the vendor to size trays. Why would I expect a proces engineer to be able to run stress right off the starting line??? If I was going to size columns and reactors I would damn sure work under the tutelage of some much more experienced.

If you want to do this, go ahead and buy the stress software. Then read your Code book, read up on Mohr's Circle and Tresca, take a course from the publisher of the softare, and get your first few models checked by an EXPERIENCED stress engineer.

 

[ChrisProcess]

Thanks for that UtilityLouie. You pretty hit the nail on the head there, in terms of what I'm looking for.
I've secured money today to purchase the B31.3 guideline, so thats a start.

The overall impression I'm getting is to go with the software route.
I think the annual license renewal with Bentley is a contentious issue, hence the reluctance to consider AutoPipe.

Considering the software option, I have a number of questions:

Can anyone offer ballpark figures for single user licenses for Caesar II, ,AutoPipe and and any others?

If we were to concentrate on correct use of the programs, then. What should we consider? I assume you need to be able to fully characterize your system in terms of fittings & equipment and operating conditions - is it a case of feeding our 3d model into the system and defining operating conditions?

We use AutoPlant for our 3d modelling which exports to the .pcf format for isometrics generation (using the ISOGEN program).
Can any stress analysis programs accept this file format, or will it be necessary to rebuild our piping model each time within the program?

Once one is proficient in use of these programs, what kind of man hours are generally involved in carrying out the analysis? (obviously this will depend on the job size....lets assume 10 lines to be analysed)

The big driver for this project is cost. Or more precisely cost savings.The cost to contract the analysis out needs to be contrasted with the cost of implementation, so the above questions are geared towards that,

Also, I realize I'm a bit rapid fire with the questions at the moment but I'm finding some useful pointers on this forum, all of which are very much appreciated.